This invention relates to a rubber composition from which solid cores and solid centers (collectively referred to as center cores) of solid golf balls and wound golf balls, and one-piece golf balls are efficiently formed to a high level of resilience, and a golf ball produced using the same and endowed with high rebound. More particularly, it relates to a golf ball-forming rubber composition which is suitable for pressure molding and capable of forming molded parts having an adequate hardness and a satisfactory initial velocity capability, and a golf ball produced using the same and endowed with high rebound.
From the past, a variety of rubber compositions were used as molding materials for center cores of solid golf balls and wound golf balls, and one-piece golf balls.
A number of proposals have been made on golf ball-forming rubber compositions. For example, JP-A 11-262544 discloses a rubber composition comprising a base rubber, a co-crosslinking agent, and other components, to which is added an organic sulfur compound having an aromatic group or an alkyl group of at least 12 carbon atoms, such as diphenyl disulfide or didodecyl sulfide.
The organic sulfur compound is blended for the purpose of slowing vulcanization down so that the rubber composition is given an appropriate vulcanization rate to enable injection molding. If this rubber composition is worked by the pressure molding process which is customarily used for the molding of rubber compositions into golf balls, productivity becomes low due to the extended vulcanization time.
An object of the invention is to provide a golf ball-forming rubber composition which lends itself to pressure molding to form center cores of solid golf balls and wound golf balls, and one-piece golf balls and is capable of forming molded parts having an adequate hardness and a satisfactory initial velocity capability without lowering productivity. Another object of the invention is to provide a golf ball produced using the rubber composition and endowed with an adequate hardness and high rebound.
It has been found that when a specific organic sulfur compound in the form of non-aromatic mercaptan or dialkyl disulfide is compounded in a golf ball-forming rubber composition comprising (A) a base rubber, (B) a metal salt of an unsaturated carboxylic acid and/or a mixture of an unsaturated carboxylic acid and zinc oxide, and (C) an organic peroxide, the composition is modified to be best suited for pressure molding without lowering the vulcanization rate, despite the blending of organic sulfur compounds. The rubber composition can be molded under substantially the same conditions as conventional compositions. This ensures that molded parts having an adequate hardness and a satisfactory initial velocity capability are produced without lowering productivity.
It has been confirmed that golf balls having center cores formed of the above rubber composition and one-piece golf balls formed of the above rubber composition are satisfactory golf balls constructed to provide an adequate hardness and high rebound.
In a first aspect, the invention provides a rubber composition for golf balls, comprising (A) a base rubber, (B) a metal salt of an unsaturated carboxylic acid and/or a mixture of an unsaturated carboxylic acid and zinc oxide, (C) an organic peroxide, and (D) 0.1 to 5 parts by weight per 100 parts by weight of the base rubber (A) of a mercaptan having the general formula: R-SH wherein R is a monovalent hydrocarbon group of 3 to 25 carbon atoms, excluding aromatic groups.
In a second aspect, the invention provides a rubber composition for golf balls, comprising the same components (A), (B) and (C) as in the first aspect, and (E) 0.1 to 4 parts by weight per 100 parts by weight of the base rubber (A) of a dialkyl disulfide having the general formula: R1-S-S-R2 wherein R1 and R2 are independently selected from alkyl groups represented by CnH2n+1 wherein n is an integer of 3 to 10.
The invention further provides a golf ball comprising a center core formed of the rubber composition defined above and a cover; and a one-piece golf ball formed of the rubber composition defined above.